The following, excerpted from my book Drunk Driving Defense, 6th ed., was kindly provided by Dr. Stefan Rose, M.D., and Dr. Kenneth G. Furton, Ph.D.:
The longer the exhaled breath, the higher the breath alcohol concentration. This is explained by the fact that gravity causes the blood to “pool” at the base of the lungs. More alcohol is contained at the base of the lungs than at the top of the lungs. The last part of an exhaled breath comes from the base of the lungs and contains more alcohol than the first part of an exhaled breath, so that the breath-alcohol concentration will be higher at the end of the breath sample than at the beginning. Therefore, the “real” concentration of alcohol in the breath sample can never be determined.